Mineral insulated electric cables



Jmu 10,1967 J MCCLEERY 3,297,818

MINERAL INSULATED ELECTRIC CABLES Filed May '7, 1965 CUPIQUS/O/V R SASTANT COMP/46750 M/NEPAZ HA7H lA/SULAT/O/V PPOTfCV/Vf Ll/V/NG Inventor A MC A Horney 3,297,818 MINERAL INSULATED ELECTRIC CABLES JamesAlexauder McCleery, Aughton, near Ormskirk,

, England, assignor to British Insulated Callenders Cables Limited, London, England Filed May 7,1965,Ser. No. 454,021

j Claims priority, application Great Britain, Dec. 31, 1964,

14 Claims. (Cl. 174-106) This invention relates to mineral, insulated cables, that is cables, comprising one or more conductors separated from a metal sheath; and from each other when there is more than (one conductor, by a compacted mineral powder. The invention is especially applicable to thermocouple cables, that is cables. in which two conductors of Y the cable are of materials such that when connected together they jjfOI'lTl a thermocouple, and especially to such Y cables intended for use at high temperatures and having 5 conductor pairs of high-nickel alloys. Examples of such alloys are nickel/ chromium used in conjunction with nickel/ aluminium, as defined in British Standard Specification 11041, the former being sold under the trade names 1 T1 and Chro'meland the latter under the trade names T2 and AhlllTl6l.

We have found that the conductors of suchcables if sheathed with, certain metals, for example, stainless steel and nickel, and heated,.for example during a welding op- .eration or during E.M.F. calibration, are subject to deterioration :caused by a form of attack penetrating from the outside surface of the metal for a short distance along j the grain boundaries. The depth of penetration is usually 3 very limited and hence the invention is especially apetchantxknown as Marbles. Reagent which consists of: Anhydrous copper sulphate g 20 1 Hydrochloric acid, cone. cc 100 Water cc 100 After etching a section of the conductor with this reagent the affected portions of the grain boundaries appear broader and more heavily marked.

We have established that the attack results in some form of internal oxidation of the conductor metal.

An object of the invention is to provide such a mineral insulated cable having a sheath of stainless steel or nickel in which theuconductor or conductors are less susceptible to suchdeterioration, which causes and will hereinafter be referred toas .conductor embrittlement.

The present invention is based on our discovery that certain metals used as sheathing materials for mineral insulated cables,,for example mild steel and certain nickel alloys, which will hereinafter be particularly defined, inhibit conductor embrittlement.

The. term mild steel is used herein and in the claims to denote :mild steel of a quality suitable for use as the sheath, :or part of the sheath, of a mineral insulated cable.

Such mild steel has a carbon content of 0.2% or less. It is the mild steelused for manufacturing cold drawn seamless tubes, see for example British Standard Specification 980 of 1950, where it is referred to as CDS 1.

United StatesPatent O Patented Jan. 10, 1967 "ice scribed in British Standard Specification 3074 under Class 14. Such alloys, as sold under the trademark Inconel, contain in addition to the ingredients specified above:

Carbon Not more than .2%. Silicon Not more than .5%. Magnesium Not more than 1.0%. Copper Not more than .7%.

Sulphur Not more than .03%.

We are not certain why the inner layer of mild steel or the specified nickel/chromium/iron alloy inhibits embrittlement of the conductors but we believe that it may either act as an oxygen getter or may protect the conductors from the migration from the outer part of the sheath to the conductors of elements that sensitize the conductors to oxidation. We consider the latter possible explanation to be more likely, especially when the inner layer is of the specified nickel/chromium/ iron alloy.

We have found that the use of a lined or composite sheath of this kind reduces deterioration of the conductor or conductors influenced by or initiated at high temperatures by the stainless steel or other metal used for the outer part of the sheath. Such deterioration is especially liable to be initiated during welding the conductors of a thermocouple together to form a hot junction and sealing the end of the sheath to enclose the hot junction.

The cable in accordance with the invention can be made by filling and drawing down a workpiece having an outer sheath-forming composite tube made by fitting a tube of stainless steel or nickel or other metal that promotes conductor embrittlement with a lining of mild steel or the specified nickel/ chromium/ iron alloy. We prefer to make the outer part of the cable sheath thicker than the inner part, since this ensures that even if melting of the sheath takes place, for example during a Welding operation, the constituents of the sheath appearing at its outer surface will remain preponderantly those of the metal required to form the outer part.

A thermocouple cable in accordance with the invention will hereinafter be described by way of example with reference to the accompanying drawing which is a crosssection of the cable.

The cable conductors 1, 2 are each of diameter .15 mm. and one is of the alloy sold under the trade name Chromel and the other of the alloy sold under the trade name Alumel. The composition of these alloys is as defined in British Standard Specification 1827, namely:

Chromel: nickel and chromium 10%; Alumel: nickel 94%, aluminium 2% and the balance silicon and manganese.

The conductors are separated from each other and from a metallic sheath 4 by compacted magnesium oxide powder 3. The sheath is of outer diameter 1 mm. and consists of a stainless steel outer layer 5 of thickness 0.125 mm. and an inner layer or lining 6 of the specified nickel/ chromium/ iron alloy of thickness 0.025 mm.

At one end of the cable the conductors are welded together to form the hot junction and the end of the sheath is sealed by welding to enclose the insulated hot junction.

Examples of stainless steels used for the outer part of the sheath of the cable in accordance with the invention are those groups known as 18/8 austenitic stainless steels (with a nominal composition of 17-20% chromium, 7- 14% nickel, the balance iron and various trace elements), 25/20 austenitic stainless steels (with a nominal composition of 20-26% chromium, 15-23% nickel, the balance iron with other trace elements) and the group known as 20/25 austenitic stainless steel (with a nominal composition of 20% chromium and 25% nickel, the balance being iron and various trace elements).

The conductors of a cable having a sheath wholly of stainless steel, showed embrittlement after 15 minutes at a temperature of 600 C., whereas conductors of a composite sheathed cable in accordance with this invention did not show any embrittlement whatsoever under similar conditions.

In addition to the advantage that the conductors are less susceptible to deterioration in manufacture and in use, thermocouple cables in accordance with the invention will,

in general, exhibit greater long term stability as far as output is concerned. Also, especially when the specified nickel/ chromium/ iron alloy is used for the inner layer, higher levels of insulation resistance are maintained at high temperatures in cables in accordance with the invention.

What I claim as my invention is:

1. A mineral insulated cable having at least oneconductor separated from a metallic sheath by compacted mineral insulation in Which the sheath is of a metal which at high temperatures promotes conductor embrittlernent and is lined with a metal selected from the group consisting of mild steel and an alloy containing as essential ingredients: not less than 72% of nickel, 1417% of chromium, and 6-10% of iron.

2. A cable as claimed in claim 1 in which the outer part of the sheath is thicker than the lining.

3. A cable as claimed in claim 1 having a sheath of stainless steel lined With an alloy containing as essential ingredients: not less than 72% of nickel, 14-17% of chromium, and 610% of iron.

4. A cable as claimed in claim 1 having a sheath of stainless steel lined with mild steel.

5. A cable as claimed in claim 1 having a sheath of nickel lined with an alloy containing as essential ingredients: not less than 72% of nickel, 1417% of chromium, and 6-10% of iron.

6. A cable as claimed in claim 1 having a sheath of nickel lined with mild steel.

7 A cable as claimed in claim 1 having two conductors which form a thermocouple pair.

8. A thermocouple cable as claimed in claim 7 having a conductor pair of high-nickel alloys.

9. A thermocouple cable as claimed in claim 7 having a sheath of stainless steel lined with an alloy containing as essential ingredients: not less than 72% of nickel, 14 17% of chromium, and 610% of iron.

10. A thermocouple cable as claimed in claim 7 having a sheath of stainless steel lined with mild steel.

11. A thermocouple cable as claimed in claim 7 having a sheath of nickel lined with an alloy containing as essential ingredients: not less than 72% of nickel, 14-17% of chromium, and 610% of iron.

12. A thermocouple cable as claimed in claim 7 having a sheath of nickel lined with mild steel.

13. A thermocouple cable as claimed in claim 7 in which the outer part of the sheath is thicker than the lining.

14. A thermocouple cable as claimed in claim 7 in which one conductor is of a nickel-chromium alloy and the other is of a nickel-aluminum alloy.

References Cited by the Examiner UNITED STATES PATENTS 3,205,296 9/1965 Davis 174-118 FOREIGN PATENTS 7,910 6/1908 Great Britain.

LARAMIE E. ASKIN, Primary Examiner,

E. GOLDBERG, Assistant Examiner. 

1. A MINERAL INSULATED CABLE HAVING AT LEAST ONE CONDUCTOR SEPARATED FROM A METALLIC SHEATH BY COMPACTED MINERAL INSULATION IN WHICH THE SHEATH IS OF A METAL WHICH AT HIGH TEMPERATURES PROMOTES CONDUCTOR EMBRITTLEMENT AND IS LINED WITH A METAL SELECTED FROM THE GROUP CONSISTING OF MILD STEEL AND AN ALLOY CONTAINING AS ESSENTIAL INGREDIENTS: NOT LESS THAN 72% OF NICKEL, 14-17% OF CHROMIUM, AND 6-10% OF IRON. 